It is known to detect particles, such as sand, moving in a flowing fluid, for example oil, with a sensor which generates a signal in response to particle impact. Such a sensor conventionally comprises a piezo-electric element coupled to the sensor housing so that collision of particles with the housing deform the element, causing the latter to generate a voltage signal which may be analysed to obtain information on the nature of the impact.
In conventional analysis techniques, the signal is amplified, unwanted frequency components filtered-out and a thresholding procedure is then applied in order to determine whether the amplitude of the signal is greater than a pre-determined threshold level. If that threshold is passed, the detection of an impact is registered in a counter.
Since the magnitude of the sensor signal is related to the force of particle impact, a series of different threshold levels may be employed in order to give an indication of the relative magnitudes of particle impacts. The energy dissipated on impact is proportional to the momentum of the particle involved, from which an indication of the mass of the particle can be obtained if the velocity of impact is known (or deduced from, for example, the flow-rate of the fluid).
An example of such an arrangement is shown in U.S. Pat. No. 4,240,287.
One disadvantage of such an arrangement is that an accurate measure of particle size is not obtained, since the various impact signals are merely sorted into a number of ranges dictated by the threshold levels.